Production of grain-oriented silicon steels



May 27, 1969 3,446,680

PRODUCTION OF GRAIN-ORIENTED SILICON STEELS C. A. CLARK ET Filed June20, 1968 IIOO SW M a mawmwm T w w wmy f SwU W Z CPu United States PatentUS. Cl. 148-112 6 Claims ABSTRACT OF THE DISCLOSURE A method ofproducing Goss texture in a steel sheet containing from 3.5% to 6.5%silicon, from 2% to 12% nickel, the nickel being present in an amountsufiicient to render the steel rollable at 100 C. and 'being correlatedwith the silicon content, such that as the silicon is increased from3.5% the amount of nickel is increased to 2%, from 0.02% to 0.06%carbon, and from 0.05% to 0.3% vanadium, the balance being iron exceptfor impurities, which comprises effecting secondary recrystalliz-ationin the steel by heating it at a temperature below that of thetransformation point of alpha iron to alpha plus gamma iron. Thesecondary recrystallization treatment also effects at least substantialdecomposition of the precipitated vanadium carbides. Said treatment maybe carried out in an atmosphere of wet hydrogen.

The present application is a continuation-in-part of our copendingapplication Ser. No. 477,688 filed Aug. 6, 1965, and now abandoned.

The present invention relates to grain-oriented steel production andmore particularly to a special heat treating process which, when appliedto silicon steels of special composition, provides grain-oriented steelscharacterized by a highly satisfactory combination of magnetic and otherproperties.

As is Well known to those skilled in the art, grainoriented siliconsteels have very valuable magnetic properties and are extensively usedas laminations in transformers and other apparatus for this reason. Inthe production of these steels, an essential step is cold rolling (aterm which includes warm rolling at, say 100 C.) to produce orientationof grains. It is commonly accepted that the optimum results are obtainedwhen 0.080 inch hot rolled sheet is cold rolled to a thickness of 0.014inch with intermediate annealing. In the cold rolling steps, the steelis deformed and in the annealing steps primary recrystallization t-akesplace at temperatures over about 400 C. If the steel is heated to a highenough temperature and contains an inhibitor of the growth of primarygrains, secondary recrystallization to give the so-called Goss textureoccurs. In this texture anedge of the cube in the body-centered cubicstructure of the steel is parallel to the direction of rolling and asimple diagonal plane is parallel to the plane of the sheet or strip.

The usual, if not the most common, inhibitor of primary grain growthheretofore employed, that is to say, the agent for facilitating theproduction of Goss texture, is manganese sulfide, and in practice,manganese and sulfur are invariably added to the steel to provide thissulfide. In practice, the secondary recrystallization is effected at atemperature of the order of about 1,2 O C. primarily because the optimumtexture is produced at this temperature.

Although the manganese sulfide is required in order to produce anadequate amount of Goss texture, it is deleterious to the magneticproperties if it remains in the steel and most of the sulfur is,therefore, removed when it has served its purpose. This is oftenaccomplished by coating 'the steel with magnesia before heating it tocause secondary recrystallization. During this heating, the manganesesulfide decomposes if the temperature is as high as about l,200 C.Substantially all the manganese goes into solution in the iron and thesulfur is removed by diffusion and combination at the surface withmagnesia while it is serving its essential purpose in promotingsecondary recrystallization. The fact that 1,200 C. is the order oftemperature required to decompose manganese sulfide is a secondaryreason for effecting the secondary recrystallization at thistemperature.

Typically, the rolled steel 0.014 inch thick is decarburized by heattreatment for five minutes at 820 C. in wet hydrogen. The steel is thencooled to room temperature and coated with magnesia and finally therecrystallization to give the Goss texture is effected in dry hydrogenat 1,200 C. and in the course of it the sulfur content is reduced to amaximum of 0.003%.

The silicon in the steel increases the electrical resis'tivity and soreduces the eddy current 'losses and it also reduces the hysteresisloss. It is known that the optimum silicon content for minimummagnetostriction is about 6.3% but air melted steels heretoforecommercially produced and containing more than about 3.5% silicon are sobrittle that they cannot satisfactorily be cold rolled. It is possibleto increase the silicon content to about 4.0% and still cold roll thesteel if the melting is performed under vacuum but, of course, thisincreases the expense of manufacture. Thus, at the present timecommercially produced, grain-oriented silicon steel normally containsabout 3.2% silicon (with a tolerance of 0.2%), not more than about0.003% carbon, not more than about 0.003% sulfur and from about 0.05% to0.15% manganese. The "balance is iron, impurities being kept at low aspossible.

Recently it has been found that it is possible to increase the siliconcontent of grain-oriented steels while still keeping the steel coldrollable by incorporating special amounts of nickel in the steels. Thisis described and claimed in US. Patent No. 3,238,073 which issued onMar. 1, 1966. It has now been found that by further modification of boththe composition of the steel and of the heat treatment, We can producesteel that, when magnetized, has magnetic properties superior to thoseof the standard silicon steels extensively used today. This involves theuse of special amounts of vanadium in siliconnickel steels and theutilization of low secondary recrystallization temperatures.

It is an object of the present invention to provide a special heattreatment which, when applied to silicon steels of special composition,results in grain-oriented silicon steels of enhanced magneticcharacteristics.

Other objects and advantages will become apparent from the followingdescription taken in conjunction with the accompanying drawing in which:

FIGURE 1 depicts a relationship between the silicon and nickel contentsof the steels;

FIGURE. 2 sets forth illustratively the iron-silicon phase diagram; and

FIGURE 3 is a graphical presentation of various curves which indicatevarious phase boundaries of steels containing certain silicon contents,the nickel content being varied.

In the steels in accordance with the present invention, the siliconcontent thereof is from 3.5 to 6.5% and there is present an amount ofnickel within the range 3 of 2% to 12% sulficient to render the steelrollable at 100 C. The amount of nickel required depends on and iscorrelated with the silicon content, increasing as the siliconincreases. The preferred correlation in which the nickel content mustvary with the silicon content for this purpose is shown in FIG. 1 of theaccompanying drawing, in which curve A shows the minimum content forthis purpose. It will be seen, for instance, that at 4% silicon thereshould be preferably at least about 3.7% nickel, and :at 5% silicon atleast 6.5% nickel. At 6% silicon, as much as 8% nickel is required foroptimum rolla'bility at 100 C. It is desirable to add no more nickelthan is required to render the steel rollable at 100 C. since an excessof nickel in comparison with silicon leads to loss of magneticproperties. We find that steels nominally containing 4% silicon giveexcellent properties and in these steels we prefer to inculde a nominalamount of 4% nickel. The tolerance in each of these percentages is 0.3%and, therefore, the preferred steels contain from 3.7% to 4.3% siliconand from 3.7% to 4.3% nickel.

An important factor in the choice of the composition of the steel is theexistence of the so-called gamma loop in the iron-silicon phase diagram.This is shown at B in FIGURE 2 of the accompanying drawing. Within theloop an iron-silicon alloy is austenitic (i.e., in the gamma form) or inthe hatched part of the loop, partly ferritic (i.e., in the alpha form)and partly austenitic. If Goss texture is to be produced, the steel mustbe wholly ferritic during the recrystallization. Now, as explainedabove, the temperature of secondary recrystallization of the nickel-freealloys is of the order of 1,200 C. or above and it will be seen thatunless the steel contains 3% or more silicon, it will come within theloop at the secondary recrystallization temperature and this is indeedthe reason why nearly all previous grain-oriented silicon steels havecontained 3.2% silicon.

In contrast to silicon, nickel is an austenite former and the additionof nickel enlarges the gamma loop. The curve C of FIGURE 2 isapproximately the loop in a steel containing nickel. When we found theapproximate shape of the loop in various nickel-containing siliconsteels, it appeared that at the necessary recrystallization temperaturethey would be at least partly austenitic and, therefore, it would beimpossible to produce satisfactory Goss texture in them. It has beensurprisingly found that it is possible to produce a satisfactory Gosstexture in a nickel-containing steel provided that the recrystallizationnecesary to produce Goss texture is effected at a temperature below theloop, i.e., below the temperature of transformation from alpha to alphaplus gamma iron.

in addition to all the foregoing, it has also been found that manganesesulfide is not very effective in promoting secondary recrystallizationto Goss texture at these lower temperatures required by the presence ofthe gamma loop in the nickel-containing steels. However, it has beenfurther found that Goss texture can be promoted by vanadium carbides,which can be eliminated from the final steel during the secondaryrecrystallization at relatively low temperature, thereby enablingoptimum magnetic properties to be achieved. Thus, our inventioncomprises first, the replacement of manganese by vanadium and thereplacement of sulfur by carbon, and, second, producing Goss texture ata temperature below the transformation point from alpha to alpha plusgamma iron on heating.

FIGURE 3 of the accompanying drawing shows graphically the way in whichthis transformation temperature varies with the nickel content in steelsof different silicon contents. The urves D, E, and F relate to 4%, 5%and 6% silicon contents, respectively. The area on the lefthand side ofeach curve is that in which the steel is ferritic. Thus, to take anexample, the 4% silicon and 4% nickel steel is ferritic up to the pointX, i.e., up to about 930 C. Although curves for only three siliconcontents are given, it will be understood that similar curves for steelsof other silicon contents can be drawn from tests on specimens of suchsteels, as will be appreciated and understood by those skilled in theart.

The speed of secondary recrystallization increases with the temperature,and it is therefore desirable that the temperature of secondaryrecrystallization should not be too low. We rather surprisingly find,however, that in the steel containing 4% silicon and 4% nickel atemperature of about 850 C., i.e., in the range of 845 C. to 855 C.,gives the best results. On the other hand, in a steel containing 6%silicon, in which the nickel content should be at least 9%, thetemperature may not exceed 830 C. if the steel is to remain whollyferritic, and we prefer not to work below 800 C. In any event, thetemperature should be at least 850 C. and advantageously does not exceedabout 950 C., e.g., not in excess of about 930 C.

Considering the composition of the steel further, manganese is present,if at all, only .as an impurity, say not exceeding 0.02%. Sullfiur isnow no longer required and, as an impurity, is kept as low as possible,say, below 0.003%. Broadly, the purer the final steel, except in respectof its essential elements, the better the final magnetic properties willbe.

To be effective, the vandaium must be present at precipitated carbideduring the secondary recrystallization. It may be precipitated in thecourse of the hot rolling and annealing steps or the steel may be heatedfor this purpose only immediately before the secondaryrecrystallization. The vanadium carbide appears from X-ray examinationto be mainly V C It is a particular advantage of the use of vanadiumthat its forms carbide readily with the carbon in the steel and thatthis carbide is readily decomposed at the relatively low temperature ofsecondary recrystallization. Other carbides which might be used asinhibitors of primary recrystallization, such as titanium carbide andniobium carbide, are stable at the temperatures in question.

The carbon content is between 0.02% and 0.06%, and preferably is 0.03%,in the initial steel. Preferably, there is enough vanadium to combinewith all the carbon but there is no advantage to be gained in includingmore vanadium than this. Having regard to these considerations, thevanadium content may be from 0.05% to 0.3% and is preferably from 0.08%to 0.15% in steels containing from 0.02% to 0.03% carbon.

Thus, in summary, the initial steel contains from 3.5% to 6.5% silicon,from 2% to 12% nickel, the nickel content being high enough to renderthe steel rollable at C., from 0.02% to 0.06% carbon and from 0.05% to0.3% vanadium, the balance being iron except for impurities. Theimpurities such as sulfur should be kept as low as possible.

Two examples will now be given.

EXAMPLE I Steel containing 4% nickel, 4% silicon, 0.015% vanadium and0.03% carbon, the balance being iron except for impurities, is hotrolletd at 1,000- C. into sheet 0.080 inch thick and annealed for 10minutes at 900 C. The sheet is then cleaned by shot-blasting. The sheetis next reduced in thickness by rolling at 100 C., the metal beingimmersed in boiling water before each pass, with intermediate annealingsteps, each comprising heating at 850 C. for 10 minutes in hydrogen,when the steel is 0.040 inch and 0.025 inch thick. In this way, thesheet is brought down to about 0.014 inch thick. The steel is nextheated in an inert atmosphere for 15 minutes at 700 C. to precipitatethe carbides. The steel is then heated in dry hydrogen for 24 hours at850 C. and in the course of this heating Goss texture is produced andthe vanadium carbide is decomposed, the vanadium going into solution inthe steel and some carbon being removed by the hydrogen. The sheet canbe further decarburized by annealing in wet hydrogen for 30 minutes at750 C.

EXAMPLE II In this example the treatment is simplified; the anneal afterthe hot rolling is eliminated and no specific annealing treatment isgiven to precipitate the vanadium carbide. Further, by the use of wethydrogen during the secondary recrystallization treatment, the need fora final decarburizing treatment to remove the carbides is eliminated.

Steel containing 4% nickel, 4% silicon, 0.10% vanadium :and 0.03%carbon, the balance being iron except for impurities, is formed by hotrolling at 1,000 C. into sheet 0.080 inch thick. The sheet is thencleaned by shot-blasting. The sheet is next reduced in thickness byrolling at 100 0., being immersed in boiling Water before each pass,with intermediate annealing steps, each comprising heating at 850 C. forminutes in hydrogen or cracked ammonia, when the steel is 0.040 to 0.025inch thick. Secondary recrystallization to Goss texture, and at the sametime carbon removal, is effected by annealing in wet hydrogen (dew pointC. to minus 10 C.) for 24 hours at 850 C.

Grain-oriented steel containing 4% silicon and 4% nickel producedaccording to the invention has high saturation induction of about 19,400gauss and has a watts loss per pound at 50 cycles per second less byabout 10% than that of nickel-free 3.2% silicon grainoriented steel. Athigh frequencies, the advantage as shown by watts loss is morepronounced.

The present invention is of particular benefit in the production ofgrain-oriented silicon steels, the magnetic characteristics of whichrender them most useful in power applications, including transformers,motors, generators, etc. A common commercial application includestransformer cores formed of laminations of the grainoriented steelscontemplated herein.

Although the present invention has been described in conjunction withpreferred embodiment, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

We claim:

1. In the production of gnain-oriented silicon steels in which secondaryrecrystallization is effected to achieve Goss texture, the method ofproducing Goss texture in a steel sheet containing from 3.5% to 6.5%silicon, from 2% to 12% nickel, the nickel being present in an amount inwhich the minimum nickel content is correlated with the silicon contentin accordance with curve A of FIG. 1 and suflicient to render the steelrollable at 100 C. from 0.02% to 0.06% carbon, and from 0.05% to 0.3%vanadium, the balance being iron except for impurities, which compriseseffecting secondary recrystallization and at least substantialdecomposition of the precipitated vanadium carbides in the steel byheating it at a temperature below that of the transformation point ofalpha iron to alpha plus gamma iron.

2. A process as set forth in claim 1 in which the steel contains 0.02%to 0.03% carbon and from 0.08% to 0 .15% vanadium.

3. A process as set forth in claim 2 in which the secondaryrecrystallization is effected in an atmosphere of wet hydrogen.

4. In the production of grain-oriented silicon steels in which secondaryrecrystallization is effected to achieve Goss texture, the method ofproviding Goss texture in a steel sheet containing from about 3.7% toabout 4.3% silicon, about 3.7% to about 4.3% nickel, about 0.02% toabout 0.06% carbon, about 0.05% to about 0.3% vanadium, with the balancebeing iron except for impurities, which comprises effecting secondaryrecrystallization and at least substantial decomposition of theprecipitated vanadium carbides in the steel by heating it at atemperature of at least about 750 C. but below the temperature at whichtransformation of alpha iron to alpha plus gamma iron occurs.

5. The method set forth in claim 4 in which the steel contains about0.02% to about 0.03% carbon.

6. The 'method set forth in claim- 4- in which the temperature ofsecondary recrystallization is from about 845 C. to about 855 C.

References Cited UNITED STATES PATENTS 2,209,684 7/1940 Crafts.

3,096,222 7/1963 Fiedler 148-111 XR 3,147,158 9/1964 Fiedler.

3,184,346 5/1965 Fiedler.

3,214,303 10/1965 Fiedler 148l11 3,238,073 3/1966 Clark 148-3155 XR3,239,332 3/1966 Goss 1481 11 XR 3,278,348 10/19'66 Foster l48110 233UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,446,680 Dated September 18, L969 Inventor(s) CHARLES ALFRED CLARK,RONALD JOHN BUTT 8: JOHN JEFFERSON MA;

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

w Column 3, line 70, for "urves" read --curves--. Column A, line 16, for"850C." read --750C.--. Column 4, line 58, for "0.015%" read "0.15%".

SIGNED AN'U SEALED M Meet:

Fletcher wnmmr E SCif-IUYLER JR 0mm 00111111551101 1151 of Patent!

